Using light for information transfer offers a series of advantages in comparison to pure electrical procedures, primarily, when high data rates are needed or long distances have to be covered. In contrast, digital processing and storage of information is today carried out almost exclusively electronically in integrated components, which are mostly produced on the basis of silicon, for in example in Complementary Metal Oxide Semiconductor Technology (CMOS). For converting information between electronic storage and processing components and optical transmission lines, electro-optical components, which may be integrated with electronic circuits, therefore are of great interest.
Today, fast electro-optical modulators are mostly integrated on substrates, which have a strong linear electro-optical effect (Pockels effect), with which substrates the refraction index may be changed for at least one polarisation by applying a voltage. Examples for such materials are lithium niobate (LiNbO3), ammonium dihydrogen phosphate (ADP), potassium dihydrogen phosphate (KDP), beta barium borate (BBO), or gallium arsenide (GaAs). The changing of the refraction index in these materials is particularly based on the non-linear polarisation of bounded electrons by the applied electric field and practically occurs instantaneously. Typically, waveguide-based components are provided in the form of stripes with low index contrast, diffused into (e.g. 0.2% for the diffusion of Ti into LiNbO3). Typically, the electric voltage is applied via supply lines, which extend on the surface of the substrate directly over the waveguide or a little bit laterally shifted parallel to the waveguides. In (order to avoid optical losses, these supply lines need to have a certain minimal distance to the optical waveguide, or, however, the part being adjacent to the wave guide has to be made of transparent material. Such an arrangement is known from the patent application publication US 2003/0053730.
These modulators have a plurality of disadvantages: On the one hand, they may be integrated only very difficultly in electronic components on a common substrate, since many substrates being usually used to implement electronic components, particularly silicon, have no or only a very poor—and therefore not being technically useable—linear eletro-optical effect.
Furthermore, waveguide curves often may only be provided with large radii of curvature, since the index contrast of the waveguides diffused into is very low. Due to the same reason, branchings of these waveguides have also be dimensioned very lengthy. Therefore, miniaturizing these components is at least difficult. Furthermore, the mode field diameter is very large due to the low index contrast, and the volume, which has to be filled by the applied electric field, is accordingly large. Therefore, comparably high voltages are needed for operating such a modulator. At the same, both electrodes are usually placed on the surface of the substrate. Therefore, only a small part of the electric flux lines flows through the light-carrying region of the optical waveguide, and only this part contributes to the electro-optical interaction. The residual electric field unnecessarily increases the stray capacitance being in effect between the electrodes, and therefore the current being necessary for operating the modulator. In total, these components therefore need high input power, which can mostly be provided by special and expensive driver electronic circuits.
Besides the components being based on the linear electro-optical effect, semiconductor-based electro-optical modulators presently are matter of intensive research. Thereby, components are investigated based on the change of the complex refraction index due to injecting or spatially concentrating free charge carriers (see US 2005/0175270, US 2004/0208454, US 2005/0089257, or Liu et al., “A high-speeded silicon optical modulator based on a metal-oxide-semiconductor capacitor”, Nature 427, page 615 ff., 2004).
The electric band width of these modulators, however, is comparably poor up to now, since either the life-time of the corresponding charge carriers acts restricting, or the velocity is relevant, with which the necessary amount of charge can be circulated.
Therefore, it is preferred to directly integrate components, which are based on fast electro-optical effects, for example the linear or the quadratic electro-optical effect, on substrates, which also are suited for implementing electronic components. For doing so, optical waveguides have to be provided with additional features not depending on the substrate material. This can be achieved by effecting a strong interaction of the guided light with suited surrounding material, for example electro-optical material. Particularly in case of combining a high-refractive waveguide core with a low-refractive surrounding material, this interaction can be increased due to the discontinuous superelevation of the normal component of the electric field at the dielectric boundary layer. The structure scientifically proposed in “Guiding and Confining light in void nanostructure”, Opt. Lett. 29 (11), page 1211 ff., 2004 by Almeida, and discussed in further publications uses this principle for a special waveguide consisting of two parallel high-refractive stripes and a surrounding material embedded in-between.
Baehr-Jones et al., “optical modulation and detection in slotted Silicon waveguides”, Opt. Express 13 (14), page 5216 ff., 2005 describes an implementation of structure, with which the waveguide stripes consist of doped silicon, and are embedded in a linear electro-optical polymer. By applying a voltage to the waveguide stripes, the refraction index in the spacing is changed, and the component may be used as electro-optical phase shifter. This component, however, has only a very small electric band width (appr. 6 MHz) not being useable in practice, since the electric supply lines only consist of a few thin silicon bars, and therefore are of very high resistance. Furthermore, producing the small gap between the waveguide stripes and filling it is technologically very demanding.
In the U.S. Pat. No. 6,993,212 B2, Block et al. describe an optical waveguide with a silicon waveguide core and an electro-optical waveguide cladding. Details concerning the dimensions of the structure are not given. Furthermore, metal is mentioned as electrode material. With metallic electrodes, typically, a relatively large minimal distance between the electrodes and the light guiding waveguide core has to be kept for avoiding optical losses. In case of the components having the same length, this in turn disadvantageously causes a relatively high operating voltage. Using the waveguide core itself as electrode is also not mentioned, and also not possible with the described metal electrodes, by implication. Furthermore, an adaptation of the group velocities between electric and optical signal, being advantageous for achieving a high band width is also not mentioned.
In US 2003/0174982 A1, Ridgway et al. describe (electrode and core arrangements for polarisation-independent waveguides. Generally, metal electrodes are used here, also. In one passage of the description transparent electrodes are even mentioned, but without more details concerning their design. Moreover, one has to assume that the material being proposed to do so, namely ITO, has a refraction index which is even a little bit higher than the refraction index of the used silica glass core so that also here an efficient light guiding demands relatively large distances between the core and the electrode, and therefore a relatively high operating voltage in case of the components having the same length. By all means, it is not a matter of a high-index contrast waveguide. Using the waveguide core as electrode is also not mentioned.